The v-Ha-ras Tg.AC mouse has been shown to develop cutaneous neoplasms in response to specific chemicals, full-thickness wounding, and UV radiation exposure, and tumor development is dependent upon activation of expression of the transgene. Previous work has demonstrated that transgene expression can be detected in a region of hyperplastic hair follicles identified by some as a putative stem cell resevoir. The goal of these studies is to utilize inducible expression of the ras transgene in Tg.AC mouse skin as a means to identify and characterize potential stem cell populations residing in the hair follicle. We have shown through the use of removal of the interfollicular epidermis through felt-wheel abrasion that transgene-expressing develop, which is direct evidence that squamous lesions arise from hair follicle origin in these mice. In addition, we have found transgene expression by in situ hybridization in re-epithelialising skin as early as 6 days following abrasion, strengthening the hypothesis that latent neoplastic cells originate from the hair follicle and express the ras transgene. In order to address the issue of what characteristics follicular stem cells possess that provides a permissive microenvironment for transcriptional activation of a transgene driven by a hematopoietic promoter, we have demonstrated that CD34, a hematopoietic stem cell marker, is expressed in a region of the hair follicle that is consistent with the location of putative stem cells, and we have shown this expression in 7 different strains of mice, including the Tg.AC. Using flow cytometry, we have sorted a CD34+:alpha-6 integrin+ population of cells from C57Bl/6, FVB/n, and Tg.AC mice. This population represents approximately 6% of the total cells, which is similar to what would be expected from a stem cell population. To address whether these cells have stem cell characteristics, cells have been placed in colony forming assays, as well as in culture designed to determine the total proliferative capacity. In order to determine if CD34+ cells are a target for chemical carcinogens both in the Tg.AC mouse and in the multistage epidermal carcinogensis model using FVB/N and C57Bl/6 strains, we propose to use a combination of flow cytometry, immunohistochemistry, laser capture microdissection, sequence analysis, and RT-PCR to determine if these cells are potentially latent neoplastic cells.